At present, the factory-prepared systems which are intended to provide watertight roofs (except the conventional roofs of clay tile, fiber-cement or metallic elements) are mainly constituted of prefabricated asphalt-based, asphalt-elastomeric or pure elastomeric impermeable films.
The factory-prepared asphalt-based and asphalt-elastomeric sheets have usually an internal reinforcement provided by polyethylene films, non-woven polyester or non-woven fiberglass. Elastomeric films, particularly the fluid-applied elastomers, do not usually contain reinforcement in their interior although some polymeric manufactured sheets do include reinforcement to provide added strength and puncture resistance to the films. These films are applied on a structural substrate (e.g. concrete slabs), sometimes regularized by cimentitious mortar. The mortar is used to create a surface free from sharp angular projection and depressions besides providing suitable slope for water flow.
Some of these materials are applied to the mud slab through previous application of an appropriate asphalt-based primer used to fix the films strongly to the mud slab substrate. The primer is applied cold, but the film is attached to the primer, often times, through a hot process, such as by means of a torch.
In order to protect the films against the deleterious effects of ultraviolet (UV) rays, some roofing materials include in one of their faces an element to impede such UV effects on the underlying asphalt-based materials. Usually, this UV shielding element comprises an appropriate elastomer. “A waterproofing laminate suitable for use in roofs, floors, or other surfaces where waterproofing is desired, comprises an elastomeric sheet secured to a modified bitumen layer and a release sheet secured to the modified bitumen layer. Certain preferred materials for used in the laminate are recited.” (U.S. Pat. No. 4,775,567). The UV shielding element may alternatively comprise crushed slate powder or a thin aluminum film facing that surfaces one side of the asphalt-based sheet.
These prefabricated sheets are often used for roofs with continued or sporadic traffic, usually necessary for maintenance or cleaning. Such facing materials do not give mechanical protection to the sheets, even though they do protect them against the incidence of ultraviolet solar rays. On the other hand, infrared rays are also reflected by the aluminum facing, improving thermal protection for the environment protected by the faced sheets.
There are also factory-prepared asphalt-elastomeric membranes, in which one face presents a self-adhesive finish and the other face receives, as in the previous case, a thin film facing of aluminum. As disclosed in U.S. Pat. Nos. 4,936,938, 5,096,759, and 5,142,837, a laminated roofing material includes an aluminum foil top sheet laminated to a polyethylene film by an ionomer resin. After the sheets are bonded together they are cooled to set the resin and an asphalt (bitumen) coating is applied to the exposed polyethylene sheet and covered with a release paper. The roofing material is applied over an underlayment to form a roof supported by conventional sheeting material.
Such a material has several applications in the building construction industry, as for example, the repair of metal roofs which leak due to oxidation and consequent perforation of the roof metallic cover. In this case, additional primers are not used because one of the material faces is already adhesive provided that the substrate is absolutely clean and dry to promote attachment.
The main disadvantage in the case of the aluminum-faced membranes resides in the low mechanical resistance of the coating on the exposed face. As the aluminum film is extremely thin (about 35 to 50 micrometers), it is subject to the damaging mechanical effects which may expose the asphalt-based portion of the membrane to the UV solar rays.
Another quite common occurrence in the usage of asphalt-based or elastomeric sheets to build construction roofs is the difficulty in locating eventual defects that could lead to the failure of water tightness. The infiltration can be caused by a flaw in lateral or longitudinal welding of the membrane strip overlaps or even by involuntary perforation in the sheet. Water penetrates through the flaw, reaches the mud slab and percolates into the interior of its porous matrix under the roofing membrane, until it finds a defect in the cimentitious substrate (e.g. a joint, a “bug hole”), making the leakage visible on the inside of the building. Most of the time, the point at which the leakage becomes visible does not coincide with the position of the failure which caused the leak. Moreover, as primer attaches the sheet firmly to the deck, if a dynamic crack appears in the substrate due to structural movements (e.g. severe climatic thermal gradients), the new joint will probably propagate to the roofing material, splitting it at this position and allowing water to enter the split.